The present invention relates generally to braiding machines and more particularly to a braiding machine which is constructed so as to include a closed loop induction heater for use in applications such as the braiding of a metal filament over an insulated core wire which is itself covered by a metal coated strip MYLAR insulating material having a heat sensitive adhesive on one surface. The heater is used in this particular application to activate the heat sensitive adhesive.
Braiding machines are well known in the art and have been used in making braids of various materials for over seventy-five years for a variety of different applications. Generally, these machines include a supply spool for holding a roll of a product such as electrical wire or string over which a braid is to be applied, a braiding mechanism for applying a braid around the product, a take-up spool for holding the product after the braid has been applied, a capstan for feeding the product from the supply spool into the braiding mechanism and then from the braiding mechanism to the take-up spool and a motor or other mechanism for driving the capstan, the braiding mechanism and take-up spool. These machines may also include an auxiliary spool for holding a roll of strip material which is fed into the braiding mechanism along with the product from the supply spool and a serving tool for serving the strip material into the braiding mechanism from the auxilary spool. The braiding mechanism usually includes upper and lower tiers of supply spools containing rolls of braiding material. These supply spools are arranged to revolve in opposite directions about a common axis with the braiding material from the lower supply spools passing over and under the braiding material from the upper supply spools and then converging and intermeshing in a tubular shaped braid around the product. The product moves through the braiding machine at a speed of about 15 feet per minute. An example of a typical braiding machine may be found in U.S. Pat. No. 1,064,407 which issued on Aug. 3, 1910 in the name of S.W. Wardwell.
Braiding machines are used extensively in the electrical wire and cable fabrication industry for applying braids of fabric or metal filament over various types of electrical conductors.
One particular application in which braiding machines are used and to which this invention is primarily but not exclusively directed is in the fabrication of a jacketed coaxial cable in which a braid of metal filament is formed over an insulated wire core which is itself covered by a served metal coated strip of mylar which has a heat activated adhesive on one side. In making this type of product, the strip of MYLAR insulating material is introduced into the braiding machine from the auxiliary spool and is placed in proper position next to the insulated wire core by means of the serving tool which is located in the braiding machine underneath the braiding mechanism. After the braid is applied, the strip of MYLAR insulating surrounds the insulated wire, overlapping on itself in a linear manner, and the braid surrounds the strip of MYLAR insulating. The product which is essentially a coaxial cable but without an external jacket is then wound on the take-up spool. Next, the product is removed from the braiding machine and brought over to an extrusion machine where a jacket, typically of plastic or rubber, is applied. The product is fed into the extrusion machine at speeds ranging from about 200 and 2000 feet per minute. As the braided product is being fed from the take-up spool into the extrusion machine it is heated by heat from a heat source. The heat source which is located in front of the extrusion machine is either a flame or an induction heater. The heat applied to the product is used to trigger the adhesive on the mylar strip and as a result seal the strip of MYLAR insulating about itself in a continuous sheave over the insulated wire core and under the braid. The product leaving the extrusion machine is a jacketed coaxial cable. In some applications, a second mylar coated metal strip and a second metal filament braid are applied to the product before the jacket is extruded. The second strip of MYLAR insulating usually does not have an adhesive layer. As can be appreciated, in both constructions (i.e. a single mylar/braid combination or a pair of mylar/braid combinations the heat from the heat source must travel through several layers of material (the number and types of layers depending on the particular construction of the product) before it actually reaches the adhesive.
It has been found that heating the product in this manner in order, to activate the heat sensitive adhesive is not entirely satisfactory. In particular, in some instances, especially when there are double layers of mylar and braid material, by the time the heat actually reaches the adhesive layer the product is well past the heat source. As a result the adhesive is very often not uniformly and/or not completely activated. Consequently, the strip of MYLAR insulating is not completely and continuously sealed about itself to form a sheave shaped configuration. When during use, the product is subsequently bent or twisted the unsealed areas become small openings which may result in energy losses within the cable. In other instances although the heat applied is sufficient to activate the entire adhesive it is so intense that one or more filaments in the braid are actually burned and break off. This creates a defective braid which cannot be used.
Another technique that has been employed in the past to activate the heat sensitive adhesive in this type of cable construction has been to heat the product with heat generated by a radiant heater as the product is being passed through the braiding machine. One problem with this arrangement is that the heat output from a radiant heater cannot be closely and quickly controlled. Consequently, during start up of the braiding machine and heater the heat applied is very often inadequate and during turn off of the heater and braiding machine the product is very often burned. This is especially important since braiding machines are very often turned on and off several times during a run to refill the braiding wire on the braiding wire spools. Another disadvantage of this arrangement is that the size of the radiant heater needed to accomplish the job in a satisfactory manner is relatively large. Consequently, because of space limitations, the heater is located next to and above the braiding machine. This, in turn, makes feeding the wire past the heater rather cumbersome and difficult. Also, radiant heaters are not very safe to use.
Accordingly, it is an object of this invention to provide a new and improved braiding machine.
It is another object of this invention to provide a braiding machine for applying a metal braid over an insulated wore core which is itself surrounded by a metallic coated strip of MYLAR insulating having a heat activated adhesive on one side.
It is still another object of this invention to provide a braiding machine which includes a heat source which can be easily and closely controlled controlled.
The above and other object are achieved according to this invention by providing a braiding machine which includes a heat source in the form of a closed loop type of an induction heater. The induction heater is mounted directly on the braiding machine, is located so that the product can be easily fed through it and is used so as to apply heat to the product after the braid has been applied and as it is being fed to the take-up spool. Since the product passing through the braiding machine is moving at a relatively slow rate and the heat source is an induction heater, the application of heat to the product can be very easily and very closely controlled. As will become readily apparent, the induction heater is limited to use with metallic type of braids.